Stellarium is free software which renders realistic skies in real time with openGL. It is available for Linux/Unix, Windows and MacOSX. With Stellarium, you really see what you can see with your eyes, binoculars or a small telescope.

Pros: Stellarium is a free and easy-to-use alternative to most commercial astronomy software applications. Stellarium is a cross-platform application for Mac, Windows, and Linux/Unix.

Cons: None significant.